1. Field of the Invention
The present invention relates to a process for preparing silanes containing halogen bonded to silicon.
2. Description of the Related Art
An important representative of the halosilanes is silicon tetrachloride (tetrachlorosilane), which is a water-clear, colorless, readily mobile liquid which has a choking odor and fumes under air. Silicon tetrachloride is used to prepare silicones, silanes and silicic esters, to obtain silicon dioxide, SiO2, and very pure silicon, and for the surface treatment of polymers and metals. A further important representative of the halosilanes is silicon tetrafluoride which can be obtained, for example, by reacting silicon dioxide with alkali metal fluoride and sulfuric acid. Unlike silicon tetrachloride, silicon tetrafluoride is gaseous at room temperature.
A series of very hydrolysis-sensitive halogen substitution products are derived from the silanes and have a similar structure to the alkyl halides, for example the chlorohydrocarbons. For example, the chlorosilanes monochlorosilane and dichlorosilane, and also tetrafluorosilane and the hydrogen-containing fluorosilanes monofluorosilane, difluorosilane and trifluorosilane, are colorless gases, while trichlorosilane and tetrachlorosilane are liquid. Also known are bromosilanes which are obtained, for example, by brominated silanes by means of tin tetrabromide.
Chlorosilanes find use as adhesion promoters, for preparing silylamines and for introducing silicon into organic compounds (silylation). Organochlorosilanes, for example methylchlorosilanes, are of industrial significance for the preparation of silicones. The remaining derivatives of the silanes, which would be formulated in a similar manner to the corresponding carbon compounds, for example silanone, are generally, with the exception of the silanols and siloxanes, so unstable that at best their organically substituted representatives have hitherto become known, for example dimethylsilanone. Since 1981, organic derivatives having Si,Si— and Si,C— double bonds (disilene, silabenzene, methylenesilane) have also become known. However, the stabilities cannot be compared with those of analogous carbon compounds.
It is known that silicon tetrachloride can be prepared by heating a mixture of calcined silica and carbon in a chlorine stream or chlorinating ferrosilicon in the presence of silicon carbide, SiC, at 500-1000° C. Carbon-free generation of silicon tetrachloride is not possible.
Similar processes find use for preparing further halosilanes.
The German patent DE 195 34 922 C1 discloses the use of microwave radiation to prepare trichlorosilane. In this known process, tetrachlorosilane is reduced in a fluidized bed reactor, in which a fluidized bed composed of silicon particles is installed in the reactor, the silicon particles are heated to a temperature of from 300 to 1100° C. by irradiation of microwave radiation into the reactor and tetrachlorosilane- and hydrogen-containing reaction gas is passed through the fluidized bed and reacted with the silicon particles to give a product gas which comprises trichlorosilane.
The German laid-open specification DE 199 48 395 A1 discloses a radiation-heated fluidized bed reactor and a process for preparing highly pure polycrystalline silicon by means of this reactor. The radiation source which finds use is one for thermal radiation, in which case the thermal radiation can be generated with the aid of microwave heating.